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7 IOP Calibration Procedures
7.10 RHMUX IOP Calibration Procedure
R688 HPM High-Performance Process Manager Service 455
December 2020 Honeywell
Calibration method
The input to the RHMUX IOP is calibrated by using two input voltages for each scale, one at 20% of full
range, and on at 80% of full scale. The RHMUX IOP will check to see if the voltage that is present is
within 20% of what is expected for each step in the calibration procedure.
If the input voltage is out of the expected voltage range, a signal with the led and in the the case of
automated factory calibration is generated. When the input voltage is digitized, the firmware will take 34
readings, eliminate the highest and lowest readings, and average the remaining readings. The gain can
then be computed from the factor: Delta Input/Delta Expected. The resultant gain constants are stored in
nonvolatile memory within the RHMUX IOP microprocessor. The field calibration factor is also set to 1.
If field calibration is performed at 50 hertz, rather than 60 hertz, the calibration coefficients have a ratio
of 545/655 prior to storage. This figure is not the exact ratio of 50 hertz to 60 hertz because of a
peculiarity in the A/D converter in the RHMUX IOP. Since calibration coefficients are stored just for 60
hertz, the firmware compensates accordingly when 50 hertz operation for the RHMUX IOP is selected.
In all cases, millivolts are computed as:
(A/D Counts - ZERO Counts) x GAIN.
GAIN is determined at calibration time as:
Factory mV/Count) x (Field Calibration Factor) x (50 Hz/60 Hz Factor)
Field calibration overview
The field calibration procedure is designed with the assumption that the RHMUX FTA may be some
distance from the control room and the console when calibration is performed. Therefore, the procedure
provides for all the interaction between the person performing the calibration and the RHMUX FTA
when the initial calibration command is executed at the console and the tasks that must be performed
locally at the RHMUX FTA.
In addition, the RHMUX IOP will not exit the calibration sequence if it is unsuccessful, but rather
remains at the failed step in the calibration sequence in order to allow it to be repeated. This means that
in order to use the RHMUX IOP again, or restart the calibration sequence, power to the RHMUX IOP
must be cycled off and then on again.